1. Technical Field
An example embodiment of the present invention relates in general to synchronization between base stations, and more particularly, to a method and apparatus for synchronization between base stations to provide broadcast multicast service in a multi-cell environment.
2. Related Art
A Long Term Evolution (LTE) mobile communication system is a packet-based mobile communication network and a next-generation communication system evolved to provide various high-speed data services. The LTE system defines a Multimedia Broadcast and Multicast Service (MBMS) based on a Multimedia Broadcast multicast service Single Frequency Network (MBSFN) in order to provide broadcast services through a wireless communication network.
In order to provide broadcast service based on the MBSFN, all base stations belonging to the corresponding service region should transmit the same information to a plurality of terminals simultaneously. For synchronized transmission, the base stations have to use the same scheduling algorithm and receive packets with the same content from the upper node before a radio transmission period starts. Also, for synchronized transmission of information, all the base stations need to be synchronized. A region in which all base stations are synchronized is defined as a MBSFN synchronized area. Also, a group of cells providing the same service, among base stations (cells) belonging to such a MBSFN synchronized area, can be defined as a MBSFN area.
For synchronized transmission of information, the base stations belonging to the MBSFN area have to receive the same scheduling information from a Multi-Cell/Multicast Coordinate Entity (MCE). A Broadcast Multicast Service Center (BM-SC) provides information using a SYNC protocol for synchronized transmission. The SYNC protocol has time stamps and packet serial numbers as additional information that is used to check scheduling times and packet loss.
Specifically, a control message for starting new broadcast service is generated by the BM-SC and transmitted to the MCE via a MBMS Gateway (MBMS-GW) and a Mobility Management Entity (MME). The MCE allocates radio resources to the new broadcast service, decides a service order based on priority, and notifies the corresponding base stations of the start of the broadcast service. Accordingly, the BM-SC which distributes broadcast contents transmits service packets for the corresponding broadcast service to the base stations, and the base stations receive the service packets and transmit them to a wireless sections.
Also, base stations belonging to a MBSFN synchronized area and MCEs controlling the base stations are accurately synchronized in hardware using a GPS or another methods so that the base stations can transmit synchronized data to the wireless sections.
As described above, although conventional synchronized base stations are configured to enable synchronized transmission using a SYNC protocol in a BM-SC, there is a problem that when the BM-SC transmits service packets for a broadcast at an arbitrary time in accordance with the start of the broadcast service, inconsistency in broadcast service time may occur. In other words, when the BM-SC transmits service packets to base stations at an arbitrary time, the service packets may reach the base stations at different times, that is, transmission delays of the service packets are generated due to different networks established between the base stations and differences in performance of intermediate routers configured between the base stations. Transmission delays cannot be avoided even when the SYNC protocol is used
Also, when it is the start of broadcast service, a service operator can designate an absolute start time of the broadcast service to base stations belonging to a MBSFN region through a management message or the like. However, such a static control method makes it difficult to properly reflect the dynamic characteristics of broadcast services, and there are also difficulties in the actual implementation.